1. Field of the Invention
The present invention relates to a light source apparatus using an electric lamp of the type called discharge lamp as a light source, which apparatus may be advantageously used in various applications and, in particular, in an illumination system for a video endoscope system.
2. Description of the Related Art
Video endoscope systems include an image sensor, such as a charge coupled device (CCD), for obtaining a scene image to be displayed on a screen of a monitor for viewing by the operator. An illumination system is required to provide an acceptable level of illumination to the scene for maintaining image quality of the scene. A typical illumination system for a video endoscope system includes a discharge lamp serving as a light source to generate light for illumination. It is desirable that a video endoscope has a light source which is capable of generating output light with high stability in its intensity. The stability in the output light intensity has two different aspects. One is that the light source should produce only small fluctuations in its output light intensity during a continuous operation, which may typically last for several hours. The other is that the light source should produce as small a decrease as possible in its output light intensity through its lifetime, which may typically last hundreds or thousands hours or more. However, gradual decrease in the output light intensity inevitably results from corresponding decrease in conversion efficiency from input electric power to output light energy of the lamp, which occurs due to several factors including progressive degradation of the electrodes of the discharge lamp. The gradual decrease in the output light intensity will mount up to a considerable fall in brightness of the discharge lamp after longtime use.
As apparent to those skilled in the art, the output light intensity level of a discharge lamp could be stabilized with ease by effecting any suitable feedback control technique to the lamp; however, a new problem would arise from this solution. Without any such control, progressive degradation of a discharge lamp will result in a considerable fall in brightness of the lamp, which can be utilized in fact as a convenient indication of the necessity for replacement of the lamp with a new one. Such indication is of particular importance for a discharge lamp used as a light source in a video endoscope system because a sudden failure of the lamp during operation of the video endoscope system is highly undesirable. Therefore, it is required that a discharge lamp used as a light source in a video endoscope system may be replaced before it terminates its life with a sudden failure, while at the same time any premature lamp replacement may be avoided for economical reasons. The latter requirement means that an appropriate indication of the necessity for lamp replacement is desired; however, the convenient indication that could be otherwise provided by a considerable fall in brightness of the lamp will be lost if feedback control is effected to the lamp in order to stabilize the output light intensity of the lamp.
Accordingly, there has been a long desire for a light source apparatus including a discharge lamp, wherein the apparatus may generate output light having high stability in its intensity, while at the same time the apparatus may provide an appropriate indication of the necessity for lamp replacement so that both a sudden failure of the discharge lamp during use of the apparatus and a premature replacement of the discharge lamp may be effectively avoided.